Levels are used by carpenters, masons, drywallers and the like for helping to assure that surfaces are horizontal or (notwithstanding their name) vertical, as the case may be. A typical level includes at least two vials, one each for ascertaining whether a surface is "level," i.e., horizontal, or "plumb," i.e., vertical.
A level vial comprises a tube made of clear glass or, preferably, clear plastic (acrylic, for example), the barrel-shaped cavity of which is partially filled with a liquid such as mineral spirits colored with a fluorescent dye. The vial is not completely filled and a bubble is thereby formed when the vial is closed. Two marker rings are applied to the outside of the vial in positions to visually divide the cavity into three portions of about equal length. Level and plumb conditions are ascertained by noting the position of the bubble with respect to the marker rings.
A problem with molding substantially-finished vials is that acrylic, the material predominantly used for making molded vials, does not stretch well. If one attempts manufacture of an acrylic vial which has thick walls (for breakage resistance), the stretch-resisting characteristic of acrylic causes breakage of core pins used in the injection molding process. On the other hand, a thin-wall vial is much more subject to breakage--builder's levels often receive rough treatment in use.
One approach (which seemingly could be used to make a thick-wall vial) is described in U.S. Pat. No. 4,347,088 (Jacquet). A barrel-shaped inner cavity is machined in a preform. Inner grooves are also machined to receive what the patent calls "reference segments" or "circlips," i.e., C-shaped rings generally like common snap rings. Of course, such rings necessarily have a discontinuity or opening about the perimeter so that the ring can be compressed radially for insertion into the cavity.
A ring is seated in each of two respective grooves and the grooves are stated to have a "semi-trapezoidal" section. It is believed that such phrase refers to a chamfer formed along one side of the groove to permit a circlip to seat fully in the groove bottom.
A problem with such construction is that the surface "transition" between a circlip and the inner wall of the tube will not be smooth, at least because of the existence of a small annular space (at the chamfer) between the circlip and the wall. In other words, a slight edge will be presented to a bubble moving along the vial and such edge may cause the bubble to "hang up" in its travel and yield an inaccurate reading.
And there may be another reason why the circlip/wall transition may not be smooth. Circlips are made to a manufacturing specification involving dimensional tolerances and their thickness, measured radially, will vary (however minutely) from clip to clip. Even with grooves that are always at exactly the same depth from tube to tube (and as a practical matter, that is a manufacturing impossibility), clips of varying radial thickness will be (a) precisely "flush" with the tube wall as desired, (b) protrude slightly beyond such wall, or (c) be slightly depressed in such wall. Either of the latter two events will also present a slight edge to a bubble moving along the vial. And in any event, the chamfer-related edge always exists to impede bubble travel.
Another problem (albeit not necessarily a major one) with known level vials involves the exteriorly-applied marker rings. Since such rings are at least somewhat exposed to contact by foreign objects, portions of such rings can be worn or chipped away over time.
An improved level vial which has a sturdy, breakage-resistant wall, which permits the vial bubble to move smoothly and freely along such wall and which is free of ring discontinuities would be an important advance in the art. Similarly, an intermediate product and a method for making such a vial would be important advances.